The present invention relates to an optical waveguide device, more specifically to an optical waveguide which can switch paths of optical signals and can deflect light, and a method for fabricating the optical wave guide device.
Optical signals, whose propagation velocity is high, make high-speed data communication possible. This makes the optical communication dominant in long-distant transmission, such as trunk communication systems. Recently, the transmission band of the optical communication has been on increase. Coupled with the development WDM (Wavelength Division Multiplex) mode, the optical communication becomes increasingly speedy and increases capacities.
To build an infrastructure of hardware of optical fiber nets of trunk communication networks, optical deflectors, which switch paths of optical signals, are necessary.
As the optical deflectors, mechanical micromirrors have been so far used. For the purpose of enabling higher integration and realizing high-speed and low-loss optical communication optical deflectors utilizing refractive index changes owing to electrooptic effect of ferroelectrics have been proposed.
As optical deflectors utilizing refractive index changes owing to the electrooptic effect of the ferroelectrics, prism domain inversion optical deflectors and prism electrode optical deflectors, for example, are proposed (Q. Chen et al., J. Lightwave Tech. vol. 12(1994) 1401, Japanese Patent Laid-Open Publication No. Sho 63-47627 (1987), etc.). These optical deflectors are formed of Ti diffused waveguides or proton exchange optical waveguides formed on LiNbO3 monocrystal substrates. In such optical deflectors, the electrodes are formed between the LiNbO3 monocrystal substrates and the optical waveguides, and an inter-electrode spacing is about 0.5 mm which is a thickness of the LiNbO3 monocrystal substrate. Accordingly, light cannot be deflected without applying high drive voltages as high as, e.g., about 600 V. Furthermore, even the application of a high drive voltage of about 600 V provides only a deflection angle of only about 0.5°; no deflection angle necessary for practical uses can be provided.
On the other hand, Japanese Patent Laid-Open Publication No. Hei-5797/1997 discloses an optical deflector using PLZT ((Pb1-xLax) (ZryTi1-y)O3), which is a ferroelectric whose electrooptic factor is high. This optical deflector includes a thin-film waveguide layer of a 600 nm-(Pb0.88La0.12) (Zr0.4Ti0.6)O3 epitaxially grown on the (100) plane of a conducting monocrystal substrate of Nb-doped STO (SrTiO3) (hereinafter called an STO substrate). This optical deflector can provide a deflection angle of 10.8° at maximum by setting an applied voltage suitably in a range of, e.g., −012 V to +12 V.
Here, in order to fabricate a practical optical crossconnection device including a large-scale optical switch having above 64 channels, it is preferable to form the above 64 optical switches on one and the same substrate. In this case, when a pitch of the channel waveguides for passing optical signals to the optical switch is 0.7 mm, the substrate must have a width of 0.7 mm×64=44.8 mm at minimum. The STO monocrystal substrate, which has good compatibility with PZT (Pb(Zr1-xTix)O3) and PLZT, is suitable for form the optical waveguides. However, The STO monocrystal substrate is very difficult to be available in a large single crystal, and is very expensive. Accordingly, the use of the STO monocrystal substrate has made it impossible to provide inexpensive optical crossconnection device with a large number of channels.
On the other hand, magnesium oxide monocrystal substrate (hereinafter called an MgO substrate) has relatively good lattice matching with PZT and PLZT. Furthermore, 4-inch φ MgO substrates can be mass-produced, and are inexpensive in comparison with the STO substrates. Then, the use of the MgO substrates will provide at low costs optical crossconnection devices, etc. having a large number of channels.
However, the expansion coefficient of MgO is 14.5×10−6/° C., which is much larger in comparison with the expansion coefficient 7.5×10−6/° C. of PZT, which is a material of the optical waveguide layer. Accordingly, heat processing of a temperature higher than 800 K for crystallizing the PZT film applies a very large stress to the PZT film, and the optical waveguide layer is broken.